Tell me the one thing which you can see, but not touch

Dr. Ambedkar has written about the time when an inspector came to his school and asked: "Tell me the one thing which you can see, but not touch.

Children give different answers.

'Sun'. 'Moon'. 'Stars'. 'Wind'.

Ambedkar said 'Paani ka ghada' and pointed to the earthen pot with water in a corner of the classroom.

(Source: I have a dream-2011)